XVIIl] LIFE-CYCLE 315 



These crithidial flagellates multiply and finally fill up large 

 portions of the gland with flagellates in all stages from typical 

 crithidial forms, attached by the flagellum, to free-swimming 

 trypanosomes closely resembling the blood type. The fly never 

 becomes infective until the trypanosomes have invaded the 

 salivary glands. The proventriculus and gut forms when 

 injected into monkeys never produce an infection. The inva- 

 sion of the glands usually occurs about the 20th day, but 

 occasionally it may take place earlier, in one case a salivary 

 gland being found infected on the 12th day. The Glossina 

 seems to become infective two to five days after the trypano- 

 somes have invaded the gland. During this period the para- 

 sites pass through the crithidial cycle, which seems to be a 

 necessary feature in the development of the trypanosome in 

 the invertebrate host before the latter can become infective. 



The whole cycle from the ingestion of the trypanosomes to 

 the appearance of young blood forms in the salivary glands 

 generally occupies from 20 to 30 days, after which the Glos- 

 sina becomes infective and probably remains so for the 

 remainder of its life. When an infected fl}^ bites an animal the 

 young trypanosomes in the lumen of the salivary glands are 

 passed down the duct together with the salivary secretion and 

 thus injected into the wound caused by the mouth parts of the 

 insect. 



Prophylaxis of Sleeping Sickness. 



In spite of the experimental evidence to shew that Glossina 

 morsiians, G. fusca, and G. tachinoides, and probably the other 

 members of the genus, are all capable of transmitting sleeping 

 sickness, the epidemiological evidence in support of the view 

 that G. palpalis is the main carrier, is overwhelming. Accord- 

 ingly, for the time being, in combating the spread of the disease, 

 the other species of tsetse-flies may be ignored. The prophy- 

 lactic measures fall into two main classes. In the first place 

 we may prevent the access of trypanosome carriers to fly areas, 

 and in the second, we may attempt the destruction of the fly. 



With regard to the former of these two methods, the diffi- 

 culties in the way of preventing the flies becoming infected 



